1. Field of the Invention
The present invention relates to a resin-molded article with a spring structure, and to a method and apparatus for preparing a superficial layer of such a resin-molded article with a spring structure. More specifically, the present invention aims to provide a resin-molded article with a spring structure which comprises the loops and curls of a three-dimensional structure such as filaments of a resin extending in a three-dimensional space, and a method for improving the quality of a superficial layer of the resin-molded article, thereby making the resin-molded article with a spring structure more adaptable to widely different applications, and adding to the value of the resin-molded article with a spring structure.
2. Description of the Prior Art
Various proposals as to the method of producing resin-molded articles with a spring structure and apparatuses suitable for the manufacture thereof have been presented.
Such proposals can be seen, for example, in Japanese Unexamined Patent Application Publications Nos. 1-207462, 1-241264, 5-106153, 7-68061, 7-68284, 7-189106, 8-74161, 8-99093, and 9-21054, Japanese Examined Patent Application Publications Nos. 3-17668 and 4-33906, and WO 01/68967A1.
Among such proposals, one advocates the use of an extrusion molding system which is equipped with shoots for guiding filaments falling from a die.
An exemplary molding system comprises two rod-like heaters arranged on the two long sides of a bundle of falling filaments (the bundle is responsible for the formation of a three-dimensional structure as described herein), and a pair of panels arranged beneath the respective heaters. Each panel consists of two strips: the upper strip forms a slope of about 45-80° with respect to a horizontal plane and the lower one is submerged under cooling water. The two lower strips can be driven centrally to enclose the bundle of filaments in a gap between them.
Another exemplary molding system comprises static or movable curved plates coated with a fluorine resin arranged along a bundle of filaments to modify the bundle into a three-dimensional structure whose density and shape can be adjusted as desired.
See Japanese Examined Patent Application Publication No. 4-33906 and or WO 01/68967A1.
The applicants of the present invention had developed a system (to be referred to simply as the pilot system hereinafter) for forming a three-dimensional structure as shown in FIGS. 12-15 during their pursuit of the present invention. The pilot system allows cooling water M to flow down over shoots 51 with which a bundle of melted continuous filaments are firstly brought into contact so as to cool the bundle of filaments 2, to thereby prevent the adhesion of filaments to the shoots 51. Each shoot 51 consists of a stainless steel plate whose working surface is coated with a fluorine resin. Coating of a fluorine resin is for preventing the adhesion of filaments 2 to the shoot 51, and promoting the spread of cooling water M over the shoot 51. Above each shoot 51 consisting of a fluorine resin-coated plate, is placed a water tank 53 for storing cooling water which has, on its bottom, a row of holes having a specified diameter being separated from each other with a specified distance. The water tanks 53 shed, like a shower, a row of cooling water M over the shoots 51. Melted filaments 2, being brought into contact with cooling water M flowing down over the shoots 51, are distorted as a result of cooling and agitation, and the filaments now deformed in loops and curls fall into a water bath 26 below.
According to a system disclosed in the Japanese Examined Patent Application Publication No. 4-33906, a bundle of filaments, when it reaches a level just above the surface of cooling water, is sandwiched by a pair of inclined upper strips which restrict the thckness of the bundle. Therefore, it may happen that parts of outward filaments fall directly on the upper strips, slip thereon and plunge into the cooling water.
A conventional system as disclosed in WO 01/68967A1 often produces resin-molded articles with a spring structure whose surfaces become rough or nappy because of defective loop or curls formation.
The pilot system also exhibits following problems, and is restricted in its improvement of the quality of products.
The cooling water does not evenly spread over the shoots 51 because of the water-repellent activity of the fluorine resin coats. The water-repellent coat hinders the uniform spread of cooling water (see FIGS. 12 and 15). Some thin water currents M independently flow down straight over the shoots 51 after they contact with the latter, while other thin water currents merge with each other. This makes it difficult to uniformly cool the entire lengthwise surfaces of a resin-molded article with a spring structure. Moreover, filaments 2 slip so readily on the fluorine resin-coated surfaces that the loop or curl formation of the filaments is disturbed (see FIG. 16(B)).
The joining distances (γ in FIG. 16(A)) required for gathering and compressing continuous melted filaments are rather short. Even if the joining distance is made large, the produced resin-molded article will have an insufficient cushion property because of the fact that the three dimensional structure is not uniformly cooled with this system.
As a consequence, the resin-molded articles with a spring structure produced by the pilot system exhibit following shortcomings.
Filaments are often cooled so much that the spring structural resin molded product produced by the system have undulated surfaces (see FIG. 15(A)). For example, on some parts of the shoots 51, water currents merge (see streak E of FIG. 14), and filaments exposed to such merged currents are cooled so much that the resin constituting the filaments shrinks and becomes less adhesive.
On the other hand, with regard to filaments exposed to independent thin currents, they are not cooled so much that their loops and curls are distorted. Thus, the filaments constituting the superficial layers 4, 5 of a resin-molded article 3 with a spring structure produced by the pilot system often have defective loops and curls. When thin water currents merge into one, there is formed a water deficient line(s) (see streak S of FIG. 14) on one or both sides of the thickened current. Melted continuous filaments exposed to such water deficient lines are not sufficiently cooled, and mechanically so fragile that, when they are pulled downward during falling, they are easily cleaved. Their cut ends spread and exhibit a characteristic pattern. When the cut ends are spiky, they take pattern (1) (see FIG. 15(B)). When the cut ends are thread-like or cord-like and stretched during falling, they take pattern (2) (see FIG. 15(C)).
Thus, the water films formed over the shoots do not have a uniform thickness, and with regard to a resin-molded article with a spring structure exposed to such water films, fusion of adjacent loops and curls is readily released (see FIG. 16(C)). This is because loops and curls have different adhesive activities owing to the uneven distribution of cooling water on the shoots. When a three-dimensional structure affected with such defects is bent, fused portions of loops and curls are easily separated giving a crush sound (see FIG. 16(C)). Such a three-dimensional structure may be defective in cushioning activity and strength.
The defective formation of loops and curls is accompanied by the distorted formation of individual filaments, resulted in changing in sectional view of the filament. If the filament is a hollow filament, for example, it will have a distorted cross-section instead of a normal round shape.
Because with the pilot system it is not possible to exert strong compression, the resin-molded article produced by the system cannot help but having a large thickness. If strong compression is simply applied using the pilot system without introducing any appropriate compensatory modifications, the resulting resin-molded article will have a reduced cushion property as well as a reduced thickness.
When the pilot system exhibiting the shortcomings as described above is used for the production of resin-molded articles with a spring structure, the resin-molded articles or products using the resin-molded article as a material exhibit following problems.
If it is required to insert a resin-molded article with a spring structure in a covering member, the undulated, nappy, or spiky surfaces of the article will get caught by the covering member, and damage the latter, or conversely fused loops forming the surfaces of the article will be pulled apart to be damaged.
Fused joints of adjacent filaments of a three-dimensional structure are susceptible to cleavage, and the cushioning property of a resin-molded article deteriorates after long use.
The resin-molded article must have a comparatively large thickness which leads to the enlargement of the volume. This causes clumsiness for handling which may lead to the increase of a production cost.
In view of above, the present invention aims to provide a system for producing a resin-molded article with a spring structure capable of uniformly distributing cooling water over the entire surfaces of shoots, thereby relieving a three-dimensional structure of the risk of being exposed either to thickened water currents or to thinned water currents, applying strong compression on the three-dimensional structure, and preventing the separation of fused loops constituting the three-dimensional structure. The present invention also aims to provide a resin-molded article with a spring structure where the surfaces are practically free from undulations, fusion of adjacent loops is prevented against separation, the cushioning property and strength are maintained even after long use, and its weight is comparatively light which is advantageous from an economical viewpoint for the material.